Rotationally specific rates of vibration-vibration energy exchange in collisions of NO(X ²II_1/2, v=3) with NO(X ²II, v=0)

Infrared ultraviolet double resonance (IRUVDR) experiments have been performed to investigate the rotational specificity of the vibrational-vibrational (V-V) exchange process, NO(X ²II^1/2,ν = 3,J_i) + NO(ν = 0)→NO(X ²II_1/2,ν = 2,J_f) + NO(ν=1), for which the vibrational energy discrepancy corresponds to 55.9 cm^-1. Radiation from an optical parametric oscillator was used to excite NO molecules into a specific rotational level (J_i) in the X²II, Ω=1/2, ν=3 state. Laser-induced fluorescence (LIF) spectra of the (0,2) band of the A ²∑⁺ -X ²II_1/2 system were then recorded at delays corresponding to a fraction of a collision. From the relative line intensities, rate coefficients were determined for transfer of the excited NO molecule from the level X ²II_1/2, ν = 3, J_i to different final rotational levels (J_f) in the X ²II_1/2, ν=2 state. Results are reported for J_i=3.5, 4.5, 7.5, 10.5, and 15.5. The data show a significant, though not strong, propensity for J to decrease by one; i.e., for ΔJ=J_f-J_i= -1, especially for the higher J_i levels. This result is interpreted as arising from a combination of (a) the tendency to minimize the energy that has to be accommodated in the relative translation of the collision partners, and (b) the favoring of ΔJ = ±1 changes when V- V intermolecular exchange occurs under the influence of dipole-dipole interactions.